Color electrophotographic process with resin deposition for stabilization of tonor image

ABSTRACT

IN AN ELECTROPHOTOGRAPHIC PROCESS FOR OBTAINING A MULTICOLOR IMAGE BY PERFORMING AN ELECTROPHOTOGRAPHIC PROCEDURE INCLUDING THE STEPS OF ELECTROSTATIC CHARGING EXPOSING, AND DEVELOPING THREE OR MORE TIMES REPEATEDLY ON A SINGLE ELECTROPHOTOGRAPHIC SENSITIVE MATERIAL, THE IMPROVEMENT CONPRISING A COLOR ELECTROPHOTOGRAPHIC PROCESS FOR ENHANCING THE MECHANICAL STRENGTH OF THE IMAGE CHARACTERIZED BY (1) PROVIDING A PHOTOCONDUCTIVE ZINC OXIDE SPECTRALLY SENSITIZED SO AS TO EXHIBIT PHOTO-RESPONSE THROUGH OUT A SUBSTANTIAL PART OF THE VISIBLE SPECTRUM, (2) PROVIDING A BINDER FOR THE ZINC OXIDE CONSISTING ESSENTIALLY OF A POLYISOCYANATE CROSS-LINKED POLYHYDROXYL RESIN WHICH FORMS A THREE-DIMENSIONAL MOLECULAR STRUCTURE, (3) CARRYING OUT THE DEVELOPMENT WITH A LIQUID DEVELOPER COMPRISING AN INSULATIVE CARIER LIQUID WITH A COMPONENT DISSOLVED THEREIN AND FINELY DIVIDED ELECTRICALLY CHARGED PARTICLES DISPERSED THEREIN ON THE SURFACE OF WHICH PART OF THE RESIN COMPONENT IS ADSORBED OR ASSOCIATED, AND (4) BRINGING THE SURFACE OF THE DEVELOPED SENSITIVE LAYER WHILE STILL WET WITH THE DEVELOPER INTO CONTACT WITH A RINSING LIQUID HAVING LESS DISSOLVING POWER FOR THE AFOREMENTIONED RESIN COMPONENT IN THE DEVELOPER THAN THAT OF THE CARRIER LIQUID THEREBY REMOVING THE DEVELOPER FROM THE NON-IMAGE AREAS AND CAUSING INSOLUBILIZATION OF THE RESIN COMPONENT WHICH DEPOSITS AROUND THE DEPOSITED TONER TO INCREASE THE MECHANICAL STRENGTH OF THE IMAGE.

United States Patent Oflice 3,689,260 Patented Sept. 5, 1972 rm. Cl. cos13/22 US. Cl. 961.2 11 Claims ABSTRACT OF THE DISCLOSURE In anelectrophotographic process for obtaining a multicolor image byperforming an electrophotographic procedure including the steps ofelectrostatic charging, exposing, and developing three or more timesrepeatedly on a single electrophotographic sensitive material, theimprovement comprising a color electrophotographic process for enhancingthe mechanical strength of the image characterized by (1) providing aphotoconductive zinc oxide spectrally sensitized so as to exhibitphoto-response through out a substantial part of the visible spectrum,(2) providing a binder for the zinc oxide consisting essentially of apolyisocyanate cross-linked polyhydroxyl resin which forms athree-dimensional molecular structure, (3) carrying out the developmentwith a liquid developer comprising an insulative carrier liquid with aresin component dissolved therein and finely divided electricallycharged particles dispersed therein on the surface of which part of theresin component is adsorbed or associated, and (4) bringing the surfaceof the developed sensitive layer while still wet with the developer intocontact with a rinsing liquid having less dissolving power for theaforementioned resin component in the developer than that of the carrierliquid thereby removing the developer from the non-image areas andcausing insolubilization of the resin component which deposits aroundthe deposited toner to increase the mechanical strength of the image.

This invention relates to a color electrophotographic process based onoverprint development.

At present, the product of Remak pty in Australia is available on themarket as a device designed to provide multi-color image on zinc oxideelectrophotographic paper by liquid development.

This process uses a metallic vat as development electrode and requires,elaborate manual operations. In this process, there is employed liquiddevelopers having electrically charged extremely fine particlesdispersed in a carrier liquid possessed of a relatively low dissolvingpower, i.e. high boiling aliphatic hydrocarbon, each charged particlebeing composed principally of a pigment particle having a minor resincomponent bonded to the surface thereof. The image to be obtained byusing such liquid developers suffers from extremely low mechanicalstrength because an excessively small resin component is bonded to thepigment primarily. When drying of the developed sheet is prompted byblowing the developing agent or rinsing liquid remaining on the surfaceof the sheet by means of a current of air or by squeezing the sheet withsqueeze rollers immediately after development, the image is readilydestroyed. Therefore, it is virtually impossible to automate such dryingoperations by use of mechanical means. The sensitive layer which isemployed in this -Remak process is prepared by combining unsensitizedwhite zinc oxide and a cross-linked thermosetting resin binder.

On the other hand, numerous office copying machines utilizing theprinciple of liquid development are available on the market. All thesemachines employ the type of sensitive paper which is so constructed asto permit the bonding agent to be slightly softened or swollen by thecarrier liquid of developing agent thereby to facilitate the fixing ofimage and which has sensitized ZnO powder dispersed in a thermoplasticbinder. Such a layer often shows a high attenuation rate of electricpotential in many nonpolar carrier liquids. Therefore, copying machinesavailable on the market avoid this problem by using, as the carrierliquid, iso-paraffinic solvents which have particularly low dissolvingpowder among the nonpolar liquids, in other words, those which haveextremely low values of solubility parameter.

As a highly effective means for enhancing the mechanical strength of thedeveloped image thereby realizing a multi-color electrophotographicprocess capable of enduring rapid treatment, there is proposed a methodcomprising developing an electrostatic latent image formed on anelectrophotographic sheet with a liquid developer containing a resinousmaterial dissolved in the carrier liquid, said carried liquid beingnon-polar but having a relatively high dissolving power, then rinsingthe developed sheet with a second liquid which is compatible with thecarrier liquid of the liquid developer, but has a far lower dissolvingpower for said resinous material than the carrier liquid. In short, thismethod employs a less active solvent for the resinous material as therinsing bath so that it serves to eliminate excessive toner and, at thesame time, to allow the resinous material to deposit as the fixing agenton the surface of image. In this case, one may surmise that suchexcessive toner will be fixed on the surface, and, consequently,background will result inevitably. It has been experimentally confirmedhowever that the toner adhering to the surface of sensitive layer byvirtue of the weak adsorbing force other than electrostatic is Washedaway completely. Further, it should be noted that the net change whichtakes place in such rinsing procedure is not such that the resinousmaterial originally dissolved in the carrier liquid abruptly separatesout of the liquid phase throughout the developed surface. The changeseems to be the enhancement of the entangling of polymer chains whichhave been loosely associated with the toner surface in the liquiddeveloper, therefore, the appearance of turbidity does not usually takeplace.

In order to reduce such method to practical use, it is necessary thatcarrier liquids possessed of a relatively high dissolving power shouldbe used in liquid developers. If this requirement is to be fulfilled,the sensitive layer is accordingly required to have a high electriccharge retaining capability even in such liquids. It has been recognizedempirically that, where a given sensitive layer is composed of aphotoconductive power and a resin binder, if the said layer is wetted byan organic liquid of such nature as to dissolve or swell the resinhinder, the electric charge on the layer leaks extremely rapidly.Consequently, it is generally most desirable to use a binder which iscured to acquire a three-dimensional molecular net-work structure.However, such efforts involve another troublesome problem. Where amulticolor image is desired directly from a multi-color original, thesensitive layer is required to have sensitivity covering the substantialportion of the visible spectrum. To meet this requirement, it may bemost desirable to employ a photoconductive element of zinc oxide havinga spectral sensitivity expanded with use of spectral sensitizer.Usually, a heavy metal catalyst is often used for curing binders. Suchcatalysts tend to destroy many spectral sensitizer, making it frequentlyimpossible to obtain a sensitive layer of stable performance. As typicalbinders, there may be cited epoxy ester resin,

alkyd resin, etc. If such binders are cured (cross-linked) in thepresence of organic acid salts of cobalt, lead, and manganese as havebeen carried out conventionally, the decomposition of the sensitizeralways proceeds during storage.

The inventors have arrived at a discovery that, as an improvement, it ismost suitable for the present method of resin curing to use apolyisocyanate. This curing method does not call for any treatment at anelevated temperature. Moreover, no decomposition of sensitizing dyes isaccompanied with the curing reaction.

Suitable polyhydroxyl compounds which can be combined withpolyisocyanate include various types of alkyd resin epoxy esters ofdehydrated castor-oil fatty acid and acrylic resins and methacrylicresins incorporated with such copolymerizable components asbeta-hydroxyethylacrylate.

The layer cured in this manner exhibits extremely high electric chargeretaining capability in various kinds of solvents possessed ofrelatively high dissolving power and therefore is highly suitable forrapid developing and during the treatments mentioned previously.

The present invention will be explained further in detail.

The present invention is an electrophotographic process for obtaining amulti-color image by repeating the steps of electrostatic charging,image exposure and development three or more times on one sameelectrophotographic sensitive layer, where the color electrophotographicprocess for enhancing the mechanical strength of the toner image ischaracterized by employing an electrophotographic layer comprisingfinely divided photoconductive zinc oxide spectrally sensitized so as toacquire photo-responsitivity throughout substantial portion of thevisible spectrum and a binder comprising a polyisocyanate crosslinkedpolyhydroxyl resin, the crosslinking being sufiicient to form a threedimensional network structure, and, in the following individual steps ofelectrophotographic procedure comprising carrying out the developmentby' using a liquid developer prepared by suspending, in an insulativecarrier liquid with a resinous material dissolved therein, finelydivided electrically charged particles on the surface of which areadsorbed or associated the said resinous material, and bringing thesurface of the developed sensitive layer into contact with a liquidhaving a sufficiently weak dissolving power for the resin component inthe aforementioned developing agent as compared with that of theaforementioned carrier liquid thereby allowing the resinous material toinsolubilize on the periphery or surface of the deposited tonerparticles.

The present invention is described in further detail 'below.

ZINC OXIDE AND SPECTRAL SENSITIZATION As is universally known, zincoxide synthesized by the French process is suitable. Zinc oxide may bemixed with a small amount of another photoconductive material for theprocess of adjusting the tone reproduction performance. Examples of suchmaterials are titanium, dioxide, cadmium sulfide, zinc sulfide, etc.

Spectral sensitization of zinc oxide is performed for the purpose ofexpanding the photo-response over the substantial portion of the visiblespectrum. The expression the substantial portion of the visible spectrumas used herein means the necessary minimum region for the natural colorreproduction.

Therefore, the photoconductor may be sensitized so as to have at leastthree peaks of sensitivity falling in the ranges of 400-460 m 450-570Ill/L, and 550-750 mg, respectively.

It is also advantageous to provide a region with an extremely lowsensitivity to impart a safelight compatibility which is a usualpractice in silver halide color printing procedure. The spectralsensitivity performance is determined from the spectral absorptionproperties of standard originals. Generally, it is a common practice toimpart peaks of photoresponse around 430440 m 520 4 540 mg, and 650-700m Such peaks are desired to be as sharp as possible. For this purpose,one may resort to the sensitization-due to J-band absorption associatedwith dye aggregate which is frequently made use of in the art of silverhalide photography.

As sensitizing dyes, there are used various kinds such as cyanine,merocyanine, hemicyanine, hemioxonol, Xanthene, sulfo phthaleine,triphenylmethane, etc. The amount of such dye to be added varies withthe purpose of use and the treatment of sensitizing material. Where aprint of high quality is required, the sensitizer is desired to bewashed off or decolorized the layer after development. Frequently, thedecoloration is accomplished by using a solvent having an organic acidor inorganic alkali dissolved therein. In this case, the composition ofthe washing bath must be selected so as not to exert any adverse effectupon the sensitive layer, the toner, the fixing agent for toner (theinsolubilized resinous material), and the electroconductive resin layerformed underneath the sensitive layer.

It, as in this case, the sensitive layer is cured with polyisocyanate,it becomes less susceptible to various kinds of organic solvents, theexpending, the freedom of selection of the composition for washing. Thisoffers a major advantage from the standpoint of practical use.

Where the amount of sensitizer added surpasses part based on parts ofzinc oxide, it is desirable to carry out decoloration. Anelectrophotographic coating with a high sensitivity for use in, forexample, printing through large magnification enlarging must beincorporated with sensitizing five times as much as the above citedconcentration.

Where the amount of sensitizer is less than part, the resulting coatinglooks weakly colored. Depending on the hue, therefore, the treatment ofdeclaration may be unnecessary.

BINDER A fairly detailed description has already been made as to thebinder. To be more specific, desirable binders are those obtained bycross-linking, with polyisocyanate compound, such alkyd resins possessedof hydroxyl group capable of reacting with isocyanate group, epoxyestersof dehydrated caster-oil fatty acid, or vinyl copolymers containingprimary hydroxyl groups.

As alkyl resins, there can be used almost all kinds which are modifiedwith drying oils, nondrying oils, styrene, acrylester, and phenolresins, etc.

The important requirement is that the acid compound thereof has anaromatic nucleus. Since aliphatic polyesters have high dependence ofresistance upon humidity, they can hardly be used for practical purpose.As aromatic acids, it is desirable to use phthalic acid and isophthalicacid. The acid content is desired to be 10-50% based on the total weightof resins. The hydroxyl value is desired to range from 10 to 100.Desired epoxyesters are those derived from dehydrated caster-oil fattyacids.

AS a series of particularly desirable compounds, there may be citedthose which have acrylester or methacrylester as the principalingredient and which have hydroxyethylacrylate,hydroxyethylmethacrylate, allyl alcohol, hydroxyethylacrylamide andother similar monomers represented by the following general formulacopolymerized with methacrylester, etc.

R CHz=( (h-CHzOH where:

R=H or CH The compounds of this series have their compositions variedwidely and can be copolymerized with acrylonitrile and styrene, etc.

Examples of commercial products include Aron SP-200l and -2002 made byToa Gosei Chemical Industry Co., Ltd.

When isocyanate is used for cross-linking, the reaction is retardedunder an acidic atmosphere. Thus, use of resins with high acid valueswill necessitate a longer curing period. Specifically, it is desired touse resins having acid values less than 25.

Desirable polyisocyanates are those which have low volatility at normalroom temperature. Typical examples are condensation products of l-moltrimethylolpropane and 3-mol tolylene-diisocyanate. As commercialproducts, there maybe cited Desmodule L made by Bayer, Coronate L madeby Nippon Polyurethane Industry, and Takenate D-l02. made by TakedaChemical Industries, Ltd., etc.

Condensation product resulting from 3 mol of xylenediisocyanate and 1mol of trirnethylol propane may also be employed.

Polyisocyanate compounds of the aliphatic family such ashexamethylenediisocyanate can naturally be used. These polyisocyanatecompounds react with alkyd or epoxyester at low temperatures andtherefore need not be roasted at particularly high temperatures. Thus,they are free from the danger of destroying the base paper or othersimilar material or spectral sensitizers. Polyisocyanate is desired tobe incorporated in an amount corresponding to 1.2 times up to severaltimes (close to times) equivalent to the hydroxyl group present in theresin (preferably more than 2 times up to about 9 times). If it isincorporated in an aquivalent amount, then the curved layer shows arelatively rapid attenuation of electric charge within a nonpolar liquidand therefore fails to meet the purpose of this invention.

Where there is employed a dry developing methodcascade, magnetic brush,or aerosol method, for example, fully satisfactory properties can beobtained by incorporating polyisocyanate in an amount 1.2-1.5 times asgreat as the equivalent amount. (In this case, the only problem to beinvolved is the dark attenuation in the air.)

However, where it is desired to use liquid development, use of a largeramount of isocyanate is required.

Where there is used a styrene-modified alkyd resin (having a hydroxyvalue of 50), for example, the dark attenuation property is purifiedkerosene or Decalin becomes comparable to that in the air only whenpolyisocyanate is used in an amount more than two times as great as theequivalent amount. When the added amount is less than the level justmentioned, the attenuation in the liquid becomes rapid, though theattenuation property in the arr may be satisfactory. If there is used aliquid developer which has an extremely rapid developing speed or a hightoner concentration, the amount of isocyanate added by amounts on theorder of 1.2-2 times as great as the equivalent amount will beallowable.

Generally, the attenuation property in the liquid for practical use isrequired to equal or approximate that of the composition of thedispersed layer in the developing agent. Measurement of attenuation isdifficult in highly volatile solvents from the practical point of view.Thus, there are used solvents which have suitable dissolving power andare dilficult to volatilize. Ordinarily there are used Decalin,kerosene, and tetralin which have been purified to the extent of showinga specific resistance of about 10 (2cm. Such a solvent is applieduniformly over an electrically charged sensitive layer so as to form athin liquid film thereon. Then, the degree of electric chargeattenuation is measured.

Experiences to date have demonstrated satisfactory correlation betweenthe attenuation property determined in the manner mentioned and theactual performance observed in liquid development. In case where rapidleakage of electric charge occurs in kerosene, any developing agentwhich uses kerosene or other carrier liquid having stronger dissolvingpower than kerosene fails to give an image of high density.

Noteworthy is a fact that, since actual liquid developing agents includeresins, ionic impurities derived from a pigment, and active agents, etc.in addition to pure Decalin or kerosene, the electric charge on thesensitive layer attenuates more rapidly in the developing agent than inpure solvent, etc. Therefore, the sensitive layer is desired to be ofsuch type that the attenuation in pure Decalin |(showing a volumeresistance of about 10 9cm.) is less than (or more than 40% in terms ofresidual ratio) for 30 second immersion. Particularly desirable level ofattenuation is less than 30% (more than 70% in terms of residual ratioresidue).

Since the present invention uses, as a carrier liquid, a nonpolarsolvent having a resistively strong dissolving power as alreadydescribed and will also be explained in further detail later on, theattenuation property of the sensitive layer in the liquid isparticularly important. As pointed out earlier, the sensitive layerwhich uses a thermoplastic resin shows rapid attenuation in Decalin andkerosene. To cite examples of such properties in reproducing sensitivepapers available on the market, the attenuation of electric charge isabout 7% of the initial electric potential (93% in terms of residualratio) after one minutes standing in the air and 5-6% after one minutesimmersion in an isoparaflin is solvent (such as isoper H made by EssoStandard Oil 00.), whereas it is as high as 36% (64% in terms ofresidual ratio) after one minutes immersion in refined kerosene. Afterone minutes immersion in refined Decalin, the attenuation is 65% (35% interms of residual ratio).

The inventors have found it necessary to use a solvent equivalent to orstronger than kerosene. This means that the speed of attenuation inkerosene must be at least substantially the same as that in the air.

The cured sensitive layer of the composition mentioned above has beenfound to meet this requirement sufliciently. It is added in thisconnection that the sensitive layer which uses, as the binder, an alkydresin or epoxy ester reisn cured by oxygen in the presence of an organicacid salt of heavy metal such as cobalt or lead as the catalyst suffersfrom rapid attenuation of electric potential in a stronger solvent andtherefore can hardly give satisfactory reproduction of continuous toneimage.

The slow attenuation in the liquid of liquid developing process is anindispersible requirement for the reproduction of continuous tone imagefor the following reason. Development proceeds preferentially in thezone having high field strength with or without development electrode.In case where a large area of uniform charge density is present in themiddle of a substantially charge-free area, development process from theedge of the changed area. As the edge portion is neutralized as aconsequence of toner deposition, the portion of high field strengthmoves gradually inward. So, development similarly moves inward. Throughthis process, a large area is finally developed uniformly. However, ifelectric charge escapes rapidly in the course of development, there willresult an image suffering from strong edge effect.

Where repulsion development (reversal development) is desired to beperformed, the latent image is developed with a toner having the samepolarity of change as the latent image while a bias voltage is appliedto the development electrode to keep substantially zero the electricfield above the maximum charge density area in the latent image wherethe toner deposition is desired.

If, the electric charge attenuates rapidly in the liquid, the optimalbias voltage varies with time and thus must be controlled following theattenuation. Such demand complicates the apparatus and gives rise tovarious problems from the standpoint of practical operation. If theattenuation in the liquid is quite slow, such troubles are eliminated.

Even if there is required adjustment of voltage at all, the width ofadjustment is small.

Another characteristic of the sensitive material of this invention isthe fact that the tone reproduction performance is prominent. This canbe demonstrated by using a liquid developer containing therein tonerparticles having diameters in the range of less than 1 1. that is,0.1-0.5}L.

v of characteristic curve in the range of 1.5-2 or more, the averagegamma of 0.7-1.8. Moreover, the maximum The determination of averagegamma, v, is made above-cited unit length so as to tangentially toughenthe the two lines as When the electrically charged sensitive plate isexposed to light projected through a photographic optical wedge, theelectrophotographic sensitive plate cured by the conentionally knowntechnique shows the average gamma,

7, of characteristic curve in the range of 1.5-2 or more, whereas thesensitive plate of the present invention shows the average gamma of0.7*l.8. Moreover, the maximum density is about 2, probably because ofthe advantage of attenuation in the liquid.

The determination of average gamma, 7, is made according to the practiceprevailing in the photographic paper industry which comprises plottingthe reflective density on the ordinate and log (amount of exposure)along the abscsisa from a developed optical wedge whereby the length ofunit density and that of log 10 are taken equal, drawing two parallellines separated by A of the above-cited unit length so as totangentially toughen the plotted characteristic curve, and designatingthe slope of the two lines as 'y.

In the ordinarily used range, the ratio of the inorganic photoconductivepowder to the binder is 100 parts of the former to 50 parts to parts ofthe latter. According to a special film forming method, a highlyinsulative film may be obtained by using the latter in the amount of 1part to 0.5 part.

LIQUID DEVELOPING AGENT Numerous pieces of literature and patentspecifications have already been published concerning the manufacturingprocesses for liquid developers. Generally speaking, a standard liquiddeveloper comprises a finely divided toner carrying a suitable amount ofelectric charge and a carrier liquid serving as the dispersing medium.For use in the present invention, the toner particle diameter should beless than 1ft. This means that the particle size of toner to bedeposited on the surface of latent image must not exceed 1 as the unit.A toner whose particle size exceeds this limit is liable to come olf thelayer surface readily during the rinsing operation. Further, theelectrophotographic characteristics of such toner are not satisfactoryin many points and therefore make it extremely difiicult to obtaindesirable results. In addition, the liquid developer is required to havea resin dissolved in the carrier liquid thereof. This is frequentlyreferred to as fixing agent. As the carrier liquid, there is generallyselectcd a highly insulative nonpolar liquid. Since the resin isrequired to be soluble in such liquid, it is restricted to aconsiderable extent. Frequently there are selected oligomers of smallmolecular weight. Suitable materials include condensation resins such aslong-oil type alkyd resins, particularly alkyd resins comprisingisophthalic acid epoxyester resins, vegetable oils, rosin-modifiedphenolformaldehyde resins, and xylene-formaldehyde resins. Occasionally,there may be used vinyl polymers such as polybutylmethacrylate,styrenebutadiene copolymer, and other styrene copolymers. Resins oflarge molecular weights are likely to form a strong film upon rinsingand therefore are difficult to use.

The resins mentioned above are soluble in cyclohexane, tetralin, Decalinand other alicyclic hydrocarbons, aliphatic hydrocarbons (includingintermixtures thereof), and these solvents incorporated therein with asmall amount of stronger solvents (such as chlorinated hydrocarbons,ketones, and aromatic hydrocarbons). Liquid developers are prepared bydispersing a finely-divided organic or inorganic pigment in carrierliquids containing such resins. There exists an opinion that thedissolved resin is strongly adsorbed on the surface of such pigment particle and behaves to control the electric charge of the pigment.However, it is difficult to observe a phenomenon which strongly supportssuch View. The major part of the resin is believed to be dissolveduniformly on the liquid phase side.

To take an example, the elcctrophoretic performance of a developercontaining a pigment having a strongtendency to be negatively chargedwill be considered. When lead chromate, which tends to assume a negativecharge, is thoroughly blended with a long-oil alkyd resin to form apaste and the paste is dispersed in cyclohexane, the dispersed particlesare charged positive due to the charge controlling capability of thealkyd resin. This fact supports that the alkyd is adsorbed on thesurface of lead chromate. After a prolonged standing, however, the resinlayer formed on the surface of resin seems to slowly dissolve out andgive rise to a negative toner. From this, it can be concluded that thepigment particle will eventually have its surface exposed so far as thecarrier liquid is capable of dissolving the resin.

Liquid developers may be prepared by a method based on another principleby using two or more kinds of resin, one of which is soluble in thecarrier liquid and the other kinds are insoluble and consequently remainpermanently deposited on the surface of the pigment. The solublecomponent and the insoluble component must be miscible with each other.The soluble component serves to ensure a stable dispersion of pigment.Such mixture of resin is blended with a pigment into a paste. When thispaste is dissolved in the carrier liquid, the consequent dissolution ofthe soluble component permits the pigment in the paste to dispersemaintaining its original dispersion state in the paste.

At this point, the insoluble component remains on the surface ofpigment. Thus, the capability of the insoluble component to control theelectrophoretic property is allowed to work stably for a long period.

Liquid developers of still another type use two or more kinds of resinscomponents, which are both soluble in the carrier liquid but which havedifferent degrees of solubility to other solvents.

When the liquid developing agent of the first or third type is appliedto the surface of latent image, the toner flows and deposits on thesurface of latent image in accordance with the change distribution.Since the toner is always accompanied with an excess carrier liquid itforms an image containing therein a homogeneously dissolved resincomponent. In the case of liquid developing agent of the second type,the pigment enclosed with resin deposits on the image portion and, atthe same time, the developed image contains a small amount of thedissolved resin.

For the present invention, it is an important requirement that the resincomponent should remain dissolved in the carrier liquid for any case.

In the subsequent step of rinsing, there must occur insolubilization ofresin. Therefore, the carrier liquid is required to have a compositioncapable of offering strong dissolving power for resins. If the carrierliquid is a paraffin solvent or fluorochlorinated hydrocarbon, thenthere will be no room for selecting a rinsing liquid having a stillweaker dissolving power. Consequently, the object of enhancing themechanical strength of image by rinsing cannot be accomplished. Thecarrier liquid is required to have the level of dissolving powersurpassing that of kerosene and preferably substantially the same asthat of cyclohexane. If cyclohexane is used alone, the evapora tion rateis too high and, consequently, there are involved various difficultiesfrom the practical point of view. Thus, there is used a solvent having asuitable evaporation rate or a mixture of a solvent of weaker dissolvingpower with 9 a solvent with a stronger dissolving power in order tosuitably control the evaporation rate.

For example, there are used kerosene, Decalin, mixture of cyclohexanewith kerosene, mixture of Decalin with kerosene, mixture of isooetanewith cyclohexane, cyclohexance containing about 3-10% of vegetable oil,Decalin containing 3-10% of vegetable oil, kerosene containing 3-15 oftetralin, isoparaffin containing -30% of tetralin, and kerosenecontaining 2-8% of toluene. Depending on the requirements of dissolvingpower and evaporation rate more complicated mixtures may be used.Generally, it is desirable to use carrier liquids having suchcompositions as will give a solubility parameter of from 7.5 to about8.8 per the room temperature. The values approximately correspond tokauri-butanol values of 33 8 Regarding concentrations or the amounts ofsuch resinous materials which are dissolved in the carrier liquid of thedeveloper liquid, and which are insolubilized by the application ofwashing liquid, 0.3 to 30 parts by weight of such material, and morepreferably 0.5 to 10 parts by Weight, may be employed per 1 part byweight of the dispersed particles in the developer.

RINSING O'F DEVELOPED IMAGE The developed electrophotographic materialbears a uniform thin layer of the developer liquid due to surfacetension, trapping to the minute structure present in the surface andother causes. Such excess toner will cause a considerable background ifthe liquid film is allowed to dry. When the image-carrying layer isimmersed in a highly insulative liquid miscible with the carrier solventbefore drying, the excess liquid which can cause background can bewashed away. As a result of experiment, it has been disclosed that theimage is destroyed during rinsing operation depending on the dissolvingcapability of the rinsing liquid to the resinous material contained inthe liquid developer. In the case of half-tone images, destruction ofthe image is difiicult to observe because the attracting force is largebetween the toner and the latent image. Destruction toner of image islikewise difficult to observe where the rinsing operation is carried outwith great care. However, when the toner image is of continuous tone, orwhen the rinsing liquid is applied to the developed surface at anincreased flow rate so as to reduce the rinsing time, image destructioncan be observed clearly. Further when the excessing liquid remaining onthe sensitive layer is squeezed by means of squeezing rollers, the imagemay be tfieformed or broken and transferred onto the roller surace.

The aforementioned problems have been noted to occur when the rinsingliquid can dissolve the resin component which has been dissolved in thecarrier liquid of the developer. It has been ascertained that thisdifficulty is overcome perfectly when the rinsing liquid exerts a poorersolubilizing activity against the dissolved resin present in the imageportion. Three types of developer have been described previously. Atthis point, attention is made to the fact that they all contain resincomponents which are dissolved in the carrier liquid. If such resinsremain undissolved in the course of rinsing, then highspeed rinsing andrapid squeezing can be permitted. If, in the liquid developing agents ofthe second type, the rinsing liquid dissolves the insoluble resindeposited on the surface of pigment particle, image destruction occursas a natural consequence.

In fact, the resins used for the second type of developer andencapsulating the pigment particles are insoluble in many non-polarsolvents (having a low value of solubility parameter) and only dissolvedby those which have a stronger dissolving power than kerosene orcyclohexane. Therefore, one need not worry about the solubilization.

Practical combinations of resin and rinsing liquid will be shownafterwards in the preferred embodiments of this invention. To cite atypical example, a liquid developer is prepared by treating an organicor inorganic pigment with a varnish obtained by cooking a rosin-modifiedphenolformaldehyde resin together with a polymerized linseed oil or astyrene-modified alkyd resin, then dispersing the treated pigment in acarrier liquid composed principally of cyclohexane whereby the varnishor resin dissolves in the solvent. A rinsing liquid is composed of aninsoparaffinic hydrocarbon a certain type of straight-chain aliphatichydrocarbon or in a fluorochlorinated hydrocarbon, each having asolubility parameter below 7.5 or kauri-butanol value below 30, whichcannot dissolve the varnish or alkyd resin.

One may fear that if solubilization of resin occurs during rinsing, thentoner might also be fixed at background areas. Experiments have shownsuch fear to be entirely groundless. Since in the background portion,pigment particles are held on the sensitive layer by rather weak forcesother than electrostatic, the toner particles are easily removed by therinsing liquid even if the insolubilization of resin occurs. In theimage portion, on the contrary pigment particles adhere fast to thesurface of layer electrostatically. Upon contact with the rinsingliquid, the soluble resin component that exists between pigmentparticles separates out of the liquid phase to deposit the surface ofpigment particles and then works to keep the particles to adhere to thesurface though with a relatively weak force. This fixing of toner imageis not extremely strong and the image may be readily removed by a localforce exerted by rubbing with a finger tip. Yet, this is sufficient sofar as the force to which the image is exposed within the apparatus isconcerned. The resin which is obtained by cooking rosin-modifiedphenol-formaldehyde resin with polymerized linseed oil is composed of acondensation product consisting primarily of the former and theunreacted linseed oil. The former component is insoluble in hydrocarbonsolvents with low KB values or in fluorochlorinated hydrocarbons and thelinseed oil is soluble in almost all solvents. Accordingly, it isbelieved to be the former that exhibits the fixing function at the timeof rinsing. Middle or long type vegetable oil-modified alkyd resins arealso illustrated as the resin insolubilized by isoparaffinic solvents.Suitable vegetable oils include linseed oil, soybean oil, safflower oil,etc.

The rinsing liquid is allowed to contain therein the toner in an amountabout 6 of that in the developing agent. This is because the latentimage has already been developed to a great extent and will no longerattract toner. In addition, when the rinsing liquid is used repeatedly,the toner will eventually accumulate to such concentration within theliquid.

The rinsing liquid will be gradually contaminated not only with tonerbut with the carrier liquid of the developer making it more and moredifficult to cause insolubilization of the resin component.

Thus a case must be taken in constructing or designing the processingapparatus to minimize the contamination of the developer into therinsing bath. Besides one may use the rinsing bath with far largervolume than the developer.

When a multi-color image is to be obtained by overprint technique, atoner image which is already formed is submerged into the subsequentdeveloping agent after the step of drying. In many cases, the carrierliquid in the second developing liquid resembles that of the firstdeveloping agent. Accordingly, there may naturally be a fear that thefirst image may be damaged in this sec-0nd development agent.Experiments have shown, however, that no such trouble occurs actually.This is probably because the first image has been fixed onto thesensitive layer or recording layer very strongly owing to the treatmentwith squeezing rollers and especially by drying.

Where there is obtained a multi-color continuous image of tone,reduction of background in each cycle of development makes a greatcontribution to the marked improvement of the quality of final print.

1 1 EXAMPLE 1 100 parts of photoconductive zinc oxide was dispersed inmethanol. The three kinds of spectral sensitizer having the followingstructural formulae were added to the resultant dispersion. The amountsof these sensitizers thus added are indicated at the side of thecorresponding formulae.

Part

Pigment 1. (sensitizer for blue hghtlo=cn on=o ON CHQGHZC O OH O Pigment2. (sensitizer for green NaO- light). I

ww v Br S O 3N8 Pilgnlllent 3. (sensitizer for red -C The substantialpart of methanol was removed by centrifugal separation. Then, afteraddition of nbutyl acetate, the remaining methanol was further removedby centrifuge. The supernatant was discarded and the resultant paste wasmixed with 28 parts of styrene-modified alkyd resin varnish (nonvolatilecontent 50%, xylol solvent, phthalic anhydride content 21%, content ofstyrene 30%, hydroxyl value and acid value 5), suitably diluted with a1:1 mixture of butyl acetate and xylol. Immediately before coating, thediluted mixture was added 8 parts of Desmodul L (a 75% ethyl acetatesolution of the condensate principally of 3-mol tolylene diisocyanateand l-mol trimethylol propane, made by Bayer of West Germany) and theresultant mixture was spread to a dry thickness of 8, on the metalizedsurface of polyethylene terephthalate film having vacuum depositedaluminum layer. The coated was dried for 15 hours in a thermostat at C.The sensitive material thus produced showed penchromatic photoresponse.The dark attenuation property in the liquid was determined as describedbelow.

blue filter and then developed with a liquid developer containing yellowtoner. In the second cycle, exposure was made through a green filter anddevelopment was performed with a liquid containing magenta toner.Finally, exposure Was made through a red filter and development wasgiven with a liquid developer containing cyan toner. The developingagents had the compositions shown below. After the step of development,the sensitive material was first rinsed with isopan E (KB value 27,solubility parameter 7.1), an isoparafiinic solvent made by EssoStandard Oil, to remove excessively adhering liquid developing agent andthen was passed through squeezing rollers so as to be deprived of allremaining liquid. The developers used were prepared as follows:

YELLOW DEVELOPER The paste obtained by mixing the following ingredients'was dispersed in 200 ml. of kerosene and 800 mol of cyclohexane.

Yellow pigment having the following structural formula CH; CH 200 0 on.c1 or a 011.0 I $=0 l C=O H H H ll 0 Varnish obtained by cookingrosin-modified pheuyl-iorrnaldehyde with linseed oil 800 Polymerizcdlinseed oil 100 MAGENTA DEVELOPER The paste obtained by mixing thefollowing ingredients was dispersed in 200 ml. of kerosene and 800 ml.of cyclohexane.

Brilliant carrnine 6B 200 Varnish obtained by cooking rosin-modifiedphenolformaldehyde resin with linseed oil 400 Polymerized linseed oil 13 CYAN DEVELOPING AGENT The paste consisting of the followingingredients was dispersed in the liquid mixture shown further down.

Microlish Blue 4G-T mg 200 Toluene ml Kerosene 250 Cyclohexane 700Linseed oil 50 When the image-bearing layer was rinsed with Isopar E andsubsequently passed through squeezing rollers, no damage was observed onthe image at all. When squeezing was given after washing withcyclohexane or kerosene, the image was broken in the course of squeezingand a part of toner image transferred itself onto the roller surface.

The cause may be explained as follows. The hydrogenated rosin ester androsin-modified phenol resin contained such as in Cyclolis Blue 4GT aresoluble in kerosene, cyclohexane, and toluene. In the developing agent,therefore, the blue pigment is dispersed withouthaving its surfacecoated with another substance. The hydrogenated rosin ester and the likeremain dissolved in the liquid phase. 011 the surface of the sensitivelayer which has undergone the step of developing, there existhydrogenated rosin ester and linseed oil as the components of liquidphase. These substances are dissolved out substantially completely intokerosene when they are washed in kerosene. Consequently, the imageportion becomes composed of pigment only and is deprived of self-fixing14 property. Thus, the aforementioned trouble occurs when the image issubjected to the action of squeezing. Incidentally, these resins areinvariably insoluble in Isopar E. Once they are washed with it, they areinsolubilized and eluted in the neighborhood of toner, enabling theimage to acquire increased mechanical strength.

EXAMPLE 2 The pigments of Example 1 were used in the same amounts, whilea binder was prepared with the following ingredients.

Parts Middle oil type linseed. oil-modified alkyl resin varnish(involatile content oil length phthalic anhydride content 33%, acidvalue 5, and hydroxyl value 20) 28 Desmodule L 8 The same colormaterials and resins of the developing agent as in Example 1 were used,except the carrier liquid was prepared with a mixture of 200 cc. ofkerosene with 800 cc. of Decalin. Using Isopar E for washing, there wereobtained satisfactory results similar to those of Example 1.

EXAMPLE 3 The same sensitizing pigments as those of Example 1 'wereused, while a binder was prepared with a mixture of 20 parts of ARON8-2001 made by Toa Gosei (involatile content 50%, hydroxyl group content2%) with 5 parts of Desmodule L. The other conditions were the same asin Example 1.

EXAMPLE 4 A pigment-adsorbed zinc oxide was prepared by the procedure ofExample 1, except the following pigments were used in the place of thoseof Example 1.

As the binder, there was used what was obtained by ture, (3) carryingout the development with a liquid derigidifying 14 parts of epoxy ester(oil length 40%) of veloper comprising, an insulative carrier liquidwith a dehydrated castor oil fatty acid with 6 parts of a conresincomponent dissolved therein and finely divided elecdensate of 3-mo1trilene-diisocyanate and l-mol trimethtrically charged particlesdispersed therein on the surface ylol propane. of which part of saidresin component is adsorbed or as- As the developing agents, there wereused the followsociated, and (4) bringing the surface of the developeding three kinds: sensitive layer while still wet with the developer intoYELLOW DEVELOPING AGENT contact with a rinsing liquid having lessdissolving power for the aforementioned resin component in the developerThe paste obtained by mixing the following ingredients than that of saidcarrier liquid thereby removing said was dispersed in a carrier liquidconsisting of 100 ml. of 10 developer from the non-image areas andcausing insolutetralin, 400 ml. of cyclohexane, and 500 ml. of Decalin.bilization of the resin component which deposits around Benzidlne yellowtype yellow pigment of the following the deposited toner to increase themechanical strength structural formula of the image.

a 01 (:1 E 250 (1:0 0:0 CHa-NH?|3CHN=N --N=N-(ilHfi-NHCH3 l O O CH3 (3H3Varnish obtained by heating rosin-modified phenyl formaldehyde resinwith linseed oil 600 Long oil type safflower oil-modified alkyd resin(oil-length 55%) 100 Polymerized linseed oil. 150

MAGENTA DEVELOPING AGENT 2. The process of claim 1 wherein said resincomponent The paste obtained by mixing the following ingredientsdeveloper 18 Varnish pregarlid cooinng a q was dispersed in a carrier 1iquid consisting of 100 m1. modified phenolformaldehyde resin 1n bolledhnseed oil.

of tetralin, 400 ml. of cyclohexane, and 500 ml. of Decalin. The claim}wherein Said @5111 1n the developer is long-oil type alkyd resins.

4. The process of claim 1 wherein the amount of the resin component inthe developer is from 0.3 to 30 parts 3 0,- OH 0 200 by Weight to onepart by weight of the dispersed particles in the developer.

on, (MM 5. The process of claim 1 wherein the solubility parameter ofsaid carrier liquid of the developer at room tem- CH3 perature is from7.5 to 8.8 and that of said liquid p0ssessed of sufficiently weakdissolving power is less than vtrnislialgtatiined biyl gealtilriilgrosn-ilmodified phenol 500 40 7.5. Th 1 h d h d d y pres See 6. eprocess of c aim 1 w erein sai 'qui possesse Polymmzed linseed on 300 ofsufliciently weak dissolving power is selected from the group consistingof iso-parafiinic hydrocarbons and fluoro- CYAN DEVELOPING AGENTchlorinated hydrocarbons.

The paste obtained by mixing the following ingredients T Process of 9 1WhCTBmPaId blnder was dispersed in a carrier liquid consisting of 100ml. of Sensltlve layer comPIlses styrene'modlfied alkyd resin andtetralin, 400 ml. of cyclohexane, and 500 ml. of Decalin.polyisocyanate- 8. The process of claim 1 wherein said binder of light-Mg. sensitive layer comprises vinyl polymer containing hy- Phthalocyaninblue 250 droxyl group and polyisocyanate. Linseed oil-modified alkylresin (oil length 52%) 550 9. The process of claim 8 wherein saidpolyisocyanate Polymerized linseed oil 200 is incorporated in an amountcorresponding from 1.2 to

10 times the amount of the hydroxyl group present in the The linseedoil-modified alkyl resin was of the type vinyl polymer. insoluble inchlorofluorinated hydrocarbon type solvents 55 10. .A method as in claim1 where after the rinsing and isoparaflin type solvents. liquid isapplied, the electrophotographic sensitive mate- In this particularexample, Daifuron S-2 (chlorofluorial is subjected to a squeezingtreatment so that a subrinated hydrocarbons made by Daikin Industries)was stantial amount of liquid remaining on the material is used as thewashing liquid. A sheet containing an image removed. of good qualitycould be obtained by passing the sheet 11. A process as in claim 1 wheresaid rinsing liquid is through squeezing rollers to remove excesswashing liquid. a volatile liquid free of non-volatile ingredients.

What is claimed is:

1. In an electrophotographic process for obtaining a Reference C emulti-colorimage by performing an electrophotographic UNITED STATESPATENTS procedure including the steps of electrostatic charging,exposing, and developing three or more times repeatedly 3,337,340 8/1967 Matkan 96-1 on a single electrophotographic sensitive material, the3,403,019 9/ 1968 Stahly et a1. 96-1.5 improvement comprising a colorelectrophotographic 3,535,244 10/ 1970 Zabiak 252-62.l process forenhancing the mechanical strength of the 3,311,490 3/1967 Fauser et a1.117-37 image characterized by (1) providing a photoconductive 3,244,5164/ 1966 Neugebauer et al 96-1 zinc oxide spectrally sensitized so as toexhibit photoresponse throughout a substantial part of the visible spec-JOHN COOPER 111, Primary Examiner trum, (2) providing a binder for saidzinc oxide consisting essentially of a polyisocyanate cross-linkedpolyhydroxyl resin which forms a three-dimensional molecular struc-96-1, 1.8, 1.7; 117-37 Ly; 252-62.1

